1. Field
The present disclosure relates to a photodetector having high photoelectric conversion efficiency with respect to incident light in a perpendicular direction.
2. Description of the Related Art
In the related art, there is known a semiconductor infrared sensor that uses a quantum well (for example, see B. F. Levine et al., “Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors”, Journal of Applied Physics, 72(9), (1992)). The quantum well infrared photodetector described in the above document converts incident infrared light to an electric current by using intersubband transition of electrons or holes. In the case of the quantum well infrared photodetector, the intersubband transition occurs only for light in an electric field vector that is parallel to the direction in which quantum wells are stacked. Therefore, such a quantum well infrared photodetector does not have sensitivity to incident light in a direction perpendicular to the substrate surface.
In contrast, Japanese Unexamined Patent Application Publication No. 2000-323694 and Hironori Nishino et al., “Quantum well infrared photodetectors”, Fujitsu. 56, 4, pp. 352-357 (2005) describe a photodetector having a grating (diffraction grating) structure for diffracting light in an oblique direction, the light being incident in the perpendicular direction. By diffracting incident light in an oblique direction, the electric vector in the direction perpendicular to the direction in which quantum wells are stacked is converted to an electric vector in the parallel direction, whereby sensitivity to the perpendicular incident light can be achieved.
Further, in the infrared sensor that uses a quantum dot or a quantum well disclosed in Japanese Unexamined Patent Application Publication No. 2012-083238, an electrode on the surface has circular holes that are arranged in a square lattice so that a surface plasmon generated on the electrode is coupled to the quantum dot or quantum well, thereby enabling the sensitivity to be increased.
However, as described in Hironori Nishino et al., “Quantum well infrared photodetectors”, Fujitsu. 56, 4, pp. 352-357 (2005), light diffracted by a grating becomes propagated light. Therefore, unless the absorption coefficient of a light absorption layer is sufficiently high, most of the converted light passes through the light absorption layer, and thus, it has been difficult to obtain high photoelectric conversion efficiency.
In addition, the configuration described in Japanese Unexamined Patent Application Publication No. 2012-083238 does not have selectivity to a polarization direction in a plane of incidence of the incident light. In order to obtain polarization characteristics, a polarization filter or the like has had to be additionally provided.